The present disclosure relates generally to Light-Emitting Diode (LED) lighting systems, and more particularly to Alternating Current (AC) driven LED lighting systems and control methods the do not introduce flickering.
Light-Emitting Diodes or LEDs are increasingly being used for general lighting purposes. In one example, a set of LEDs is powered from an AC power source and the term “AC LED” is sometimes used to refer to such circuit. Concerns for AC LED lighting systems include manufacture cost, power efficiency, power factor, flicker, lifespan, etc.
FIG. 1 demonstrates AC LED lighting system 10 in the art, which, in view of electric circuit, simply has a LED module 12 and a current-limiting resistor 14. The LED module 12 consists of two LED strings connected in anti-parallel. The AC LED lighting system 10 in FIG. 1 requires neither an AC-DC converter nor a rectifier. Even though a DC voltage is also compatible, an AC voltage VAC is typically supplied to power the AC LED circuit 10 directly. Simplicity in structure and low-price in manufacture are two advantages the AC LED lighting system 10 provides. Nevertheless, the AC LED lighting system 10 can only emit light in a very narrow time period in each AC cycle time, suffering in low average luminance.
FIG. 2 demonstrates another AC LED lighting system 15 in the art. Examples of the AC LED lighting system 15 can be found from U.S. Pat. No. 7,708,172. The AC LED lighting system 15 employs full-wave rectifier 18 to rectify an AC voltage VAC and provide a DC output power source across an input power line IN and a ground line GND. A string of LEDs are segregated into LED groups 201, 202, 203, and 204, each having one or more LEDs. An integrated circuit 22 as an LED controller has pins or nodes PIN1, PIN2, PIN3, and PIN4, connected to the cathodes of LED groups 201, 202, 203, and 204 respectively. Inside integrated circuit 22 are channel switches SG1, SG2, SG3, and SG4, and a current controller 24 as well. When the rectified input voltage VIN at the input power line IN increases, current controller 24 can adjust the conductivity of channel switches SG1, SG2, SG3, and SG4, to make more LED groups join to emit light. Operations of integrated circuit 22 have been exemplified in U.S. Pat. No. 7,708,172 and are omitted here for brevity.
FIG. 3 illustrates the waveforms of signals in FIG. 2 when the AC input voltage VAC has a sinusoidal waveform. The upmost waveform in FIG. 3 shows the rectified input voltage VIN at the input power line IN. The second shows the total number of illuminating LEDs, meaning the number of LEDs that are illuminating. The four following waveforms regard with LED currents ILED4, ILED3, ILED2 and ILED1, which, as shown in FIG. 2, refer to the currents flowing through LED groups 204, 203, 202 and 201, respectively. The total number of illuminating LEDs rises or descends stepwise, following the increase or decrease of the rectified input voltage VIN. When the rectified input voltage VIN increases, LED groups 201, 202, 203, and 204, one by one according to a forward sequence, join to illuminate. For example, when the rectified input voltage VIN increases to just exceed the forward voltage VTH1, the voltage required for driving the LED group 201 to illuminate, the LED group 201 starts illuminating. When the rectified input voltage VREC decreases, LED groups 201, 202, 203, and 204 darken, one by one according to a backward sequence. If, for example, the rectified input voltage VIN just falls below the forward voltage VTH4, the voltage required for driving all the LED groups 201, 202, 203 and 204 to illuminate, then the channel switches SG3 and SG4 are switched ON and the channel switches SG2 and SG1 are OFF, such that the LED group 204 stops illuminating, leaving only the LED groups 201, 202 and 203 to emit light. The AC LED lighting system 15 enjoys simple circuit architecture and, as can be derived, good power efficiency.
There in FIG. 3 however shows a dark period TDARK when no LEDs illuminate, because the rectified input voltage VIN is too low to drive the LED group 201. If the rectified voltage VIN is a 120-Hertz signal, the voltage valley where the rectified voltage VIN is about zero volt appears at 120 Hz, causing the dark period TDARK to show up at the same frequency of 120 Hz. This phenomenon is sometimes referred to as flickering. Even though flickering might not be perceivable by human eyes, it is reported that people watching objects exposed under the luminance of the LED lighting system 15 could feel dizzy or discomfort. It is desired to have an AC LED lighting system that produces no flickering.